Dentists frequently carry out shaping and polishing of materials such as dental restorative resins using detachable abrasive disks which are mounted on a mandrel driven by a powered rotary dental tool. Two commercially available detachable disk and mandrel combinations are commonly used, these two combinations ordinarily being referred to as the "Moore's Brass Center Disk and Mandrel" (apparently named after the principal manufacturer, E. C. Moore Co., Inc.) and the "Pinhole" disk and mandrel.
The "Moore" type brass center disk and mandrel employs a circular abrasive disk having a central hub with a square central hole. The hub is generally made by inserting a cylindrical brass eyelet into a hole in the abrasive disk and staking the brass eyelet using a suitable punch. The staked eyelet serves to reinforce the disk and provide a rotationally locked fastening means between disk and mandrel. The disk is mounted on a live mandrel having an end or arbor which is square in cross-section, axially bisected by a slot, and shaped and sized to dimensions sufficient to lockingly engage the square hole in the disk hub. "Moore" brass center disks and mandrels are well known to dentists and are illustrated in publications such as "Moore's Disk Sample Chart" (manufacturer's literature). A "Moore" type brass center disk hub is also shown in U.S. Pat. Nos. 478,881 and 4,055,897.
"Moore" brass center type disks and mandrels are widely used by dentists because disk replacement is relatively easy. However, the disk must be oriented so that the square hub in the disk is aligned with the square end on the mandrel in order to attach the disk to the mandrel. Also, a "Moore" brass center disk and mandrel must be used with care, as the end of the mandrel projects beyond the face of the mounted disk, and this projecting end has aggressive edges. If the rotating projecting end of the mandrel inadvertently strikes a dental restoration, a black mark can be left on the surface of the restoration. Also, the impact of the rotating projecting end against a dental restoration or tooth surface can cause objectionable patient pain and sensation. In addition, the edge of the disk hub can inadvertently strike a dental restoration or tooth surface even if the end of the mandrel does not do so, and the hub can thereby mark the surface of the restoration or cause patient pain.
Another drawback to the "Moore" brass center disk and mandrel is that the physical dimensions of the hub constrain the ultimate minimum useful disk diameter. The square hole in the hub is approximately 3 millimeters on each side, and the hub is generally 6 to 7 millimeters in diameter. Commercially available "Moore" type brass center disks are generally no smaller than about 13 millimeters diameter, and such disks are generally too large to be conveniently used in some closely confined dental work such as repair of cervical erosions, and Class I and Class II restorations of the occlusal anatomy of posterior teeth.
The pinhole disk and mandrel, the other commonly used detachable disk and mandrel combination, employs a disk having a central hole of approximately 1.7 millimeters diameter. No reinforcing hub is used in such a disk. The pinhole disk is mounted by placing the disk on the shaft of a truss-head or fillister-head machine screw which screw is then screwed into an axially threaded hole in the pinhole mandrel. The pinhole mandrel is provided with a shoulder against which the disk and machine screw bear. The binding action of the screw locks the disk in place. Pinhole disks and mandrels are also well known to dentists and are illustrated, for example, in U.S. Pat. Nos. 302,952, 336,695, and 1,506,078.
The pinhole disk and mandrel must also be used with care, as the protruding machine screw can accidentally strike the surface of a dental restoration or tooth, with attendant marking of the surface of the dental restoration and/or patient discomfort. An additional disadvantage of the pinhole disk and mandrel is that disk replacement is relatively cumbersome, requiring removal and rethreading of the machine screw each time a disk is replaced. Also, the ultimate minimum useful disk diameter for a pinhole disk and mandrel is constrained by the physical dimensions of the machine screw. Commercially available pinhole disks, like "Moore" brass center disks, are generally about 13 millimeters diameter or larger, and such pinhole disks are therefore too large to be used in some closely confined dental work such as repair of cervical erosions and Class I and Class II restorations of the occlusal anatomy of posterior teeth. Some pinhole disk manufacturers will supply custom made pinhole disks in diameters less than 13 mm for use by dentists who are willing to grind down the pinhole mandrel machine screw head to a smaller diameter. Disk replacement is still somewhat cumbersome with such a modified mandrel, and the disk may have a tendency to slip under load due to the reduced machine screw head size.
Other detachable abrasive disk and mandrel systems described for dental use are shown in U.S. Pat. No. 1,599,298 (in which a mandrel having a machine screw fastening means is provided with a cup shape locking screw having incorporated therein a helical locking spring) and U.S. Pat. No. 3,858,368 (in which a disk attached to a flexible sleeve is provided with a square axial hole in the sleeve, in order that the disk may be detachably mounted on an arbor having a square cross-section).
Also, in U.S. Pat. No. 3,789,462 there is shown a flexible dental polishing unit which is provided with an undercut indented bore having a non-circular cross-section, the polishing unit being mounted on a shaft having a non-circular cross-section corresponding to the bore of the polishing unit.
Other patents relating to detachable abrasive or polishing disks or mandrels are U.S. Pat. Nos. 1,786,320, 3,307,300, and 3,833,998. These patents describe detachable abrasive disks and mandrels useful for large scale polishing and abrasive equipment such as auto body polishers and disk sanders.
All of the above references specifically refer to the need for providing some positive, static locking means which would prevent rotation of the abrasive disk on the mandrel, whether the mandrel is in use (i.e. spinning) or merely at rest (i.e., not spinning). Locking means employed in the above references include disks having square or non-circular central holes mated with mandrels having a corresponding square or non-circular cross-section, as well as locking screws, springs, and the like. The use of such locking means requires care in alignment when the disk is placed on the mandrel. Also, locking means which project aggressively beyond the working face of the disk can be difficult to use in dental applications, because the locking means can strike the surface of a dental restoration or tooth during use. In addition, use of locking means which are square or non-circular in cross-section adds to the cost of manufacture of the disk and mandrel.